X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
1999-06-01
2001-01-23
Porta, David P. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Absorption
C378S053000
Reexamination Certificate
active
06178222
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to fabrication techniques of optical devices using doped optical fibers and more in particular to methods of testing the doping profiles in the core of an optical fiber.
BACKGROUND OF THE INVENTION
Active optical fibers, typically doped with erbium, are more and more often used in communication systems. In particular the use of erbium doped active optical devices, and specially of erbium doped fiber amplifiers doped is rapidly growing because of the peculiar passband and gainband characteristics of these devices that favorably coincide with the wavelengths of so-called third window, in the vicinity of 1.5 &mgr;m, which is more and more used in optical fiber communication systems.
One of the parameters that most affect doped optical fibers remains the concentration and distribution within the vitreous matrix, generally silicic, of dopant ions, in particular within the core of the optical fiber.
The dopant ions act as active centers similarly to what happens in lasers and therefore their distribution inside the matrix is fundamental in determining the performance of the active optical device.
In optical devices such as fiber amplifiers, the control of the distribution of dopants in the glassy matrix of the core is even more critical because in an optical fiber amplifier the smoothing (levelling) effect of the electromagnetic field due to optical feedback from cavity mirrors does not exist, whereas such effect does exist in lasers.
The article “Chlorine concentration profiles in silica fibers” by H. Hanafusa et al., ELECTRONIC LETTERS, Feb. 16, 1984, UK, vol. 20, no. 4, pages 178-179, and the article “Investigation of the structure of perform materials and fiber-optical waveguides utilizing quartz glass doped with germanium and boron”, by A. N. Gur'yanov et al., KVANTOVAYA ELEKTRONIKA, Moskva, October 1979, USSR, vol. 9, no. 10, pages 1238-1242, describe methods for characterizing the doping of the core of an optical fiber and of a preform, respectfully, by x-ray microanalysis. These methods do not provide a direct and reliable distribution pattern of dopant ions on a plane normal to an optical fiber axis.
The determination of the concentration profile, that is, a significatively precise characterization of the doped optical fiber continues to represent a serious problem in view of the difficulty to reliably make such a characterization and this represents a non negligible drawback for the commercial evolution of these technologies.
Until now the characterization is carried out in two different fabrication steps of a doped optical fiber. A first characterization is undertaken on a rod or preform having a diameter of a few centimeters, before the material is drawn in a continuous small diameter fiber. The characterization is made inductively, by exciting the material with a laser and measuring the intensity of the reemission.
However, the drawing process takes place at a temperature of about 800° C. and this inevitably determines a certain diffusion of the doping material during this manufacturing step. Therefore, it is necessary to repeat the characterization on the finished optical fiber, also in this case inductively, and to adapt consequently the length of the active optical fiber in order to attain the required gain value requested by the specifications.
SUMMARY OF THE INVENTION
It is evident the need for a direct characterization method (non inductive) capable of precisely define the doping of an optical fiber core.
A method has now been found, and this is the object of the present invention, to solve this so far elusive problem of directly and reliably analyze the real distribution of dopant ions on a plane normal to the optical fiber, capable of providing a reliable quality control information on the doped optical fiber, as produced, and therefore its intrinsic gain and passband characteristics.
It has now been found that is possible to obtain an optical image of the dopant distribution on a cut fiber portion by means of a soft X-ray Contact Microscopy technique (SXCM).
According to a crucial aspect of the invention it is necessary to prepare sample slices of the optical fiber, thickness of which should be smaller than 20 &mgr;m, preferably 10 &mgr;m or even less.
With such a sample thickness, the soft X-ray contact microscopy technique can attain reach a resolution of about 50 nm and, by use of proper emission targets and/or of a filter having an appropriate spectral window, it is possible to enhance the contrast between the matrix of silicic glass and, specifically for the particular dopant ions of the core.
According to the method of the invention, the distribution of the relevant dopant ions may be precisely mapped and their total concentration precisely estimated.
According to an important aspect of the invention, the cutting of sample slices with a thickness (or length) in the vicinity of 10 &mgr;m, is carried out using a Focused Ion Beam apparatus (FIB).
This type of equipment is described in the article: “Focused Ion Beam”, by Jon Orloff, Scientific American, Oct. 1991, pages 74-79.
These equipments are commercially available and they are used in microelectronics generally to perform local modifications or corrections in already fabricated devices, by being able to perform precise cuts and eventually permit to deposit conductive layers of a specific geometry. These equipments are based upon the capacity to generate a beam of strongly accelerated ions that is made to impinge on the sample. The ion beam, which can be precisely focused and guided by means of electrostatic lenses, incides the sample m -aterial being able to dislodge (sputter) ions, electrons and atoms of bombarded surface. Therefore, an FIB may also be used similarly to an SEM, though with inferior resolutions.
The most important characteristic of an FIB is the intrinsic capacity of remarkably restrict the scanning area of the ion beam on the sample surface so to notably increase the density of ion-sample impacts and consequently to attain a major transfer of energy to the sample from the incident particles on an extremely small area.
This energy generates a local microplasma and a consequent ablation of sample material within the impact area of the beam, practically producing obtaining a well defined cavity in the material being bombarded.
The ion beam can be collimated in the order of ten nanometers so that very precise cuts may be performed.
The cutting depth depends solely on the total dose (number of ions per surface unit) of ions impacted onto the scanned zone.
Typically, the ions constituting the primary beam are simply ionized ions of gallium. Gallium possesses only two isotopes with about the same isotopic abundance and is a liquid at ambient temperature. These characteristics permit to obtain a substantially monochromatic and highly energetic beam, because the ionic mass is relatively large, about 70 AMU, and a typical accelerating field is of about 25-30 KeV.
According to a further important aspect of the invention, with a plasma laser source of soft X-rays capable of generating a beam of X-rays with a sufficiently ample diameter, several samples may be simultaneously radiographed, by arranging the samples over an X-ray photoresist layer, disposed on an appropriate support, for example on the bottom of a sample holder which may have a cylindrical shape with a diameter and a height of few millimeters.
A top cover of the cylindrical sample holder may be of silicon nitride (Si
3
N
4
), of about 0.3 to 3 mm thickness. The silicon nitride top cover defines a suitable spectral window through which irradiate the samples placed onto the photoresist support of (e.g. the bottom of the sample holder), thus performing a filtering of the soft X-ray (spectral window), which enhances the contrast between the matrix and the dopants in the irradiated fiber samples.
The sample holder does not need to be evacuated.
A suitable source of soft X-rays may be a plasma laser developed at the Rutheford Appleton laboratory (RAL), Central Laser Facil
Milani Marziale
Savoia Claudio
Sottocasa Emilia
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Galanthay Theodore E.
Kiknadze Irakli
Porta David P.
STMicroelectronics S.r.l.
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